According to Robert W. Whittlesey, Sebastian Liska and John O. Dabiri over at the California Institute of Technology in Pasadena, wind farms could generate significantly more power if they were more fish-like.

 

Wind farms traditionally use the horizontal axis (HAWTs). But by doing so, the maximum power a rotor can extract is proportional to the cube of the windspeed. More specifically, the power coefficient for HAWTs is defined as the mechanical power output of the turbine divided by the power of the free-stream air through the cross-sectional area of the rotor normal to the free-stream direction.

 

But since they slow the air passing through them, neighboring turbines need to be around 10 turbine diameters apart. This is a huge limit on what can be generated from a given area of land. 

 

 

Turbines on a vertical axis (VAWTs) can actually do better than their horizontal peers when closely packed together. It is due to the aerodynamic effect called a Karman vortex street. The example below shows cloud patterns downwind of the Juan Fernandez Islands off the coast of Chile.

The repeating vortices in an air stream around a body are relatively common. And it's that same science that keeps schools of fish synchronized. It also reduces the total propulsive power needed per fish. In the field of fluid dynamics there have been well-developed models for simulating this phenomenon. This effect also reduces the fuel consumption of vehicles traveling in a platoon.

 

Studies have shown that aerodynamic interactions between a pair of counter-rotating VAWTs contribute to a higher power coefficient per turbine. An increase of up to 4 percent in the average turbine power coefficient of an array of VAWTs due to steam-tube contraction (induced higher flow speeds around neighboring VAWTs). But they never considered the possibility of alternating the directions of rotation of neighbouring VAWTs.

 

So the Caltech students designed a model to simulate the vortices of a 16 x 16 wind farm of vertical axis turbines and study the effects of closely packing them together. As it turns out, the additional acceleration that the vortices add to the wind can increase the power of vertical axis turbines. But they still do not compare with horizontal axis turbines. That said, they can be packed much more closely therefore requiring much less land.

 

The win here is that it increases the power that can be generated from any given slice of land, and it's not a small amount. "These configurations significantly reduced the land use for vertical axis wind turbine wind farms, resulting in array power density increases of over one order of magnitude compared to operational horizontal axis wind turbine wind farms," say the Caltech boys.

 

A full order of magnitude increase in power density — not bad! Caltech has already filed patents for the design.

 

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Source: Technology Review

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